1. Field of the Invention
The present invention relates to a switching power supply unit being configured to produce switching output obtained by switching DC input voltage at an output winding of a voltage conversion transformer, and a voltage detection circuit for use in such a switching power supply unit.
2. Description of the Related Art
Generally, a hybrid electric vehicle is equipped with a low voltage battery of outputting a low DC voltage such as about 12 V as a power supply for driving in-vehicle instruments such as a wiper, a headlight, a room light, an audio instrument, an air conditioner, and various measuring instruments, and equipped with a high voltage battery of outputting a high DC voltage such as about 400 V as a power supply for driving a motor. Typically, such a low voltage battery is charged by rectifying an AC output voltage from an AC generator driven by using rotation of an engine to obtain a high DC voltage, and converting such a DC input voltage into a relatively low DC voltage by using a switching power supply unit, and then supplying the low DC voltage to the low voltage battery. The high voltage battery is charged by the DC input voltage supplied from an engine side. For example, as described in Japanese Unexamined Patent Publication No. 2003-259637, the switching power supply unit performs voltage conversion by temporarily converting a DC input voltage into an AC voltage using an inverter circuit, then transforming the AC voltage by using a voltage conversion transformer and converting into a DC voltage again by using a rectifier circuit or the like.
When the DC input voltage supplied from the engine side exceeds a withstanding voltage of an internal circuit of the switching power supply unit, the internal circuit may be broken. Therefore, it is important to monitor the DC input voltage at any time to prevent the internal circuit from being broken. This is applicable not only to the switching power supply unit installed in the hybrid electric vehicle, but also to a typical switching power supply unit.
For example, in Japanese Unexamined Patent Publication No. 2003-33015, a voltage detection circuit for detecting a DC input voltage is provided. The voltage detection circuit detects a voltage induced in an output winding of a voltage conversion transformer, then estimates the DC input voltage by calculation.